Itachi explicitly said "Lets hope you'll never have to use it." This suggests it has to be some sort of last-ditch technique that Naruto should never expect to use until Sasuke pins him in a corner. Because of this, I highly doubt it would be something more sustained, like a sharingan or whatever.

Then again, what do I know? Kishimoto already knocked us over the head with this fake EMS shit that Sasuke has...clearly he has his own story-telling conventions, and doesn't afraid to alter the naruto universe to his liking, either. (Kind of reminds me of a certain Bleach series I've known).